Short wavelength fluorescent light source

ABSTRACT

Devices providing 270-320 m Mu and preferably 280 m Mu fluorescent light are described in which a primary source of shorter wavelength light is incident upon an elongated thalliumactivated alkali halide fluorescent crystal having rounded ends serving as lenses to collimate fluorescent light emanating from said ends. Methods and means are also described for prolonging the useful life of the fluorescent crystal, including optical means for filtering out the deleterious wavelengths of the light from the primary source, means for dissipating heat from the crystal to reduce its degrading effect, and a method and means for cleaning and maintaining the crystals free of surface contamination adversely affecting its fluorescent properties.

United States Ti atent 1 Allington 1 June 3, 1975 [54] SHORT WAVELENGTHFLUORESCENT 3,054,013 9/1962 Johnson 313/108 R LIGHT SOURCE 3,147,2269/1964 JonCk l 252/3013 3,200,254 8/1965 Luik, Jr. et a1. 356/207 X [75]Inventor: Robert W. Allington, Lincoln, Nebr.

[73] Assignee: Instrumentation Specialties Primary ExaminerDavis L.Willis Company, Lincoln, Nebr. Attorney, Agent, or Firm-Vincent L.Carney [22] Filed: July 24, 1970 211 Appl. No.: 64,863 [57] ABSTRACTRelated Us. Application Data Devices providing 270-320 mg. andpreferably 280 DM ion f S r N 654 303 J 26 1967 my. fluorescent lightare descr bed 1n wh1ch a primary abansdonec? 5 g i artof source ofshorter wavelength. light s incident upon an No 590 OM27 1966 abandonedp elongated thallium-activated alkali halide fluorescent crystal havingrounded ends serving as lenses to collimate fluorescent light emanatingfrom said ends. 58 Field of Search 250/71 77 88 so s4- the useful Mefluorescent Crystal mfludmg 313/25 108 109 5 tical means for filteringout the deleterious wave- 356/181 lengths of the light from the primarysource, means for dissipating heat from the crystal to reduce itsdegrad- [56] References Cited ing effect, and a method and means forcleaning and maintaining the crystals free of surface contaminationUNITED STATES PATENTS adversely affecting its fluorescent properties.2,783,407 2/1957 Vierkotter 313/109 2,901,647 8/1959 Thomas et a1.313/25 14 Claims, 4 Drawing Figures PATENTEUJUN3 1975 INVENTOR. ROBERTW. ALLINGTON ATTORNEYS SHORT WAVELENGTH FLUORESCENT LIGHT SOURCE Thispresent application is a continuation-in-part of my earlier co-pendingUS. patent application Ser. No. 590,071, filed Oct. 27, 1966 and nowabandoned for short Wavelength Fluorescent Light Source.

BACKGROUND OF THE INVENTION A source of short wavelength ultravioletlight of about 280 p. wavelength has numerous applications as iswell-known to those skilled in the art, and is particularly useful inphotometric apparatus for the measurement of optical density or lightabsorbance of protein and amino acid solutions. One method of providingsuch a light source would be to employ a thermally incandescent body inconjunction with a suitable monochromator. This method suffers from thedisadvantage that is difficult to maintain a solid body at a temperaturesufficiently high to provide a strong radiant flux at 280 ,a wavelength.Another approach to the problem would be to select a strong spectralline from the emission line spectrum of an excited element. In order toprovide a low cost, stable light source, having a wavelength of about280 p. and a practical level of intensity, it would be necessary toselect an element for excitation having both a strong emission line inits spectrum at about the desired wavelength and a low vapor pressure.Unfortunately, no element having both of these properties exists.Consequently, it has previously been the practice to obtain 280 twavelength light either either by using a thermally heated body as notedabove or by the use of a continuous-spectrum hydrogen discharge lamp inconjunction with a suitable monochromator. Such systems suffer from thedisadvantage that only a small portion of the total energy output of thesource is of the desired wavelength, and consequently the remainingunused greater proportion is wasted as heat. It is apparent, therefore,that the need has existed in the art for a low cost, easily operable,long lasting source of light of about 280 [.L wavelength.

BRIEF DESCRIPTION OF THE INVENTION This invention relates to novel shortwavelength fluorescent light sources and, more particularly, to lightsources of this type comprising thallium-activated alkali halidefluorescent crystals, to improved crystals of this type, and to methodsand means for preserving the fluorescent properties of such crystals.More specifically, the novel short wavelength fluorescent light sourcesof the invention include a crystal of thalliumactivated alkali halidewhich produces fluorescent light over a wavelength range of about270-320 ,a when irradiated by shorter wavelength ultraviolet light froma primary source included in the device. In a specific embodiment of theinvention, a single large, elongated, thallium-activated alkali halidecrystal having rounded ends is employed; the rounded ends acting aslenses to collimate the large proportion of the fluorescent light whichemanates from these ends. Other embodiments of the invention include atransparent heat shield interposed between the primary source ofultraviolet light and the crystal, and cooling means associated with thecrystal to prolong its operable life by dissipating ,degrading heatemanating from the primary light source. Optical filtering means arealso included insuchembodiments of the invention to filter out certainportions of the light in the primary source, particularlly 185 a light,which has a tendency to degrade the crystal.

In still another embodiment of the invention the novel short wavelengthfluorescent light source includes an elliptical reflector; the elongatedcrystal being disposed with its axis passing through one focal axis ofthe reflector and the bright spot of the primary light source coincidingwith the other focal axis of the reflector. In this embodiment thecrystal is disposed in a sealed tube filled with inert gas. The tube maybe of any suitable material completely transparent to the desired light,such as quartz, for example, if an improved crystal of the invention isused. However, if a crystal of previously available type is used thetube may be composed of a material which transmits light having awavelength longer than about 220 ,u and whicih absorbs light having awavelength shorter than about 220 ,u, thus filtering out any deleterious185 [.L light from the primary source. A transparent heat shield is alsodisposed between the primary source and crystal, if necessary.

The invention also includes improved thalliumactivated alkali halidecrystals containing 10' to 10 mole fraction of thallium ion with respectto potassium ion. I have discovered that such crystals, when preparedaccording to known methods, have a tendency to be degraded by heat andlight of certain wavelengths, and, therefore, the heat dissipating meansand means for filtering out harmful components of the light in theprimary source, as noted above, are included in embodiments of theinvention employing such crystals. I have made the further unexpecteddiscovery; however, that such crystals are extremely sensitive tosurface contamination and this discovery has led to the development ofother important embodiments of the invention, i.e. improved crystals ofgreatly prolonged useful life, a method for the manufacture of suchcrystals by removing surface contamination of ordinary crystals, andmeans for protecting the improved decontaminated crystals againstrecontamination. More specfically thallim-activated alkali halidecrystals are preferably washed in hexane, dried, rewashed in methylalcohol, dried again, placed in sealed protective tubes and spaced fromthe sides of the tube by inert spacing elements such as rings composedof Teflon (Trademark of El. Du Pont De Nemours Co. forpolytetrafluoroethylene) and surrounded by an inert atomsphere, such asdry argon. It has been found that such decontaminated and protectedcrystals are not adversely affected by 254 ,u. light or even 185 p.light at temperatures up to F. Therefore, when such tubed crystals areemployed in the light sources of the invention it is possible in somecases to eliminate the heat dissipating and optical filter elementsrequired in embodiments employing crystals prepared according topreviously known techniques, the surfaces of which crystals are normallycontaminated. While the above washing method employing nonaqueoussolvents is preferred, the invention also includes the use of aqueoussolvents having solvent action for the alkali halide to cleanse thesurface of such crystals.

BRIEF DESCRIPTION OF THE VIEWS OF THE DRAWINGS In the drawings in whichlike reference characters designate like parts in the several views:

FIG. 1 is a top plan view of apparatus made in accordance with anembodiment of this invention, including light filtering and heatdissipating means;

FIG. 2 is a side elevational view of the apparatus of FIG. 1, with theoptical filters and flow cells omitted;

FIG. 3 is a top plan view apparatus made in accordance with anotherembodiment of this invention, certain parts being broken away and othersshown in cross-section;

FIG. 4 is a side elevational view of the apparatus of FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION Referring now to FIGS. 1 and 2, amercury-vapor lamp 10, a primary light source, is disposed proximate toan elongated rod 11 comprising a single crystal of thallium-activatedalkali halide. The crystal is formed by growing it from a fuzzed melt ofa mixture of suitable salts, for example, thallium chloride andpotassium chloride. Because of the marked tendency of the thallium ionto remain in the melt instead of being in corporated into the potassiumchloride crystal lattice, a large excess of the thallium chloride abovethat desired in the crystal, must be present in the melt, for example,to obtain 10 mole fraction, thallium chloride is used in a proportion ofabout one part per thousand of potassium chloride by weight. Anotherreason for using a large excess of the thallium salt is because thalliumchloride, for example, boils at atmospheric pressure at a temperatureslightly below the crystal growing temperature, which temperature is ofthe order of 785795C. I have found it is important that the thalliumconcentration in the potassium chloride crystal be about to 10" molefraction of thallium ion with respect to the potassium ion. Morespecifically, if the thallium mole fraction is greater than 10-*, thewavelength of maximum emission of fluorescent light is shifted to longerwavelengths away from the desired 280 p. value, and the crystal becomesphysically weak and difficult to machine and also degrades more rapidlyin use. On the other hand, if the thallium mole fraction is less than10*, the fluorescent emission of the crystal is weakened.

The crystals can be grown by any of several known methods, as, forexample, by the Kyropoulos method, described in The Growing of Crystals,by Menzies and Skinner, Discussions of the Faraday Society, Vol. 5,306-312, (1949). However, I modify this method by using a platinumholder for the seed crystal to prevent contamination by nickel assuggested in the cited article. Also the seed crystal holder issupported by Haynes tubing, which is used to cary cooling water to theseed crystal holder, since the Haynes alloy obviates the contaminatingcorrosion sometimes experienced from stainless steel used by the citedauthors.

The mercury-vapor lamp l0 radiates ultraviolet light rich in lighthaving a wavelength of 254 a, causing the crystal 11 to radiatefluorescent light having a wavelength band of about 270-320 a. When along, narrow crystal is used, as shown in FIGS. 1 and 2, a largepercentage of the flourescent radiation will emanate from the endsthereof. In order to collimate such end radiations, the ends of thecrystals are rounded to formlenses, as shown.

Since thallium activated alkali halide crystals prepared according topreviously known methods are degenerated by heat, when such crystals areemployed, a transparent heat shield 12 is interposed between theexciting lamp and the crystal. Further, a suitable cooling means 13 anda heat transfer member 14 may be employed to advantage. The member 14comprises a block material having good thermal conductivity, such asaluminum, and the cooling means 13 may comprise a conventionalthermoelectric cooling machanism with a suitable heat sink.Alternatively, water jacketing, forced air, etc., may be used forcooling the crystal. Such heat dissipating means may also be used inconjunction with the improved crystals of the present inventiondescribed below, but due to the improved stability of such crystals, arenot ordinarily necessary unless the crystal is exposed to temperaturesabove F.

In apparatus for the measurement of optical density or light absorbanceof solutions, the fluorescent radiations emanating from the ends of thecrystal 11 are passed through flow cells 15 and optical filters 16 whichabsorb radiations having wavelengths of 290-320 1.4.. A filter of thistype is disclosed and claimed in my US. Pat. No. 3,243,595, issued Mar.29, 1966, and entitled, Compound Optical Filter Providing a SharpCut-off.

In the embodiment of the invention shown in FIGS. 1 and 2, only arelatively small fraction of the total light of the primary source lamp10 strikes the crystal. Also, when used in this way crystals prepared bypreviously known methods lose their ability to fluoresce strongly afterprolonged exposure to the exciting lamp. 1 have found that with respectto previously available crystals, the latter effect is at leastpartially due to light produced by the lamp of a wavelength other thanthat which excites the desired fluorescence of the crystal. Specificaly,light of a spectral line wavelength of u degenerates the flluorescentproperties of ordinary thallium-activated alkali halide crystals anddoes not contribute significantly to the fluorescent intensity of suchcrystals. As a matter of fact, even the desired spectral line wavelengthof 254 1. which produces the desired fluorescence has a tendency todamage previously available crystals, although to a lesser degree thanthe 185 p. light. Both of these spectral lines are present in thespectrum of the excited mercury vapor in the lamp 10. Therefore, whenusing a crystal of the type previously available it is desirable tointerpose between the lamp and the crystal a filter which will absorbthe undesirable 185 p. wavelength. One suitable filter for this purpose,which transmits wavelengths longer than 220 p. and absorbs wavelengthsshorter than 220 ,u, is composed of a clear, highsilica glass sold underthe tradename Vycor 7910, by Corning Glass Works. The filter can beincorporated in the apparatus in a number of ways as, for example, bymaking the envelope of the mercury-vapor lamp of this material, byinterposing a sheet of the material between the exciting lamp and thefluorescent crystal, or by enclosing the fluorescent crystal within atube made of the material. It is here pointed out that a filter made ofthis material is also useful in conjunction with ordinarythalliumactivated crystals when such crystals are excited by a sourceother than a mercury-vapor lamp, for example, a hydrogen lamp whichproduces a continum of wavelengths in the ultraviolet region.

Reference is now made to FIGS. 3 and 4, which show another embodiment ofthe invention in which a greater proportion of the output of the primarylight source is utlized. The primary light source, mercury vapor lamp10, is positioned so that its bright spot coincides with one focal axisof an elliptical reflector 20.

The axis of the crystal 11 passes through the other focal axis of thereflector. The crystal is enclosed within a sealed tube 21. The tube 21may be made of any suitable material completely transparent to thedesired light, such as quartz, for example, if it is to enclose one ofmy improved crystals which do not require protection from 185 p. lightin the source. On the other hand, if a previously available crystal isused, the tube 21 is made of the abovementioned material which transmitsrays having a wavelength longer than 220 p. and absorbs rays having awavelength shorter than 220 41.. Preferably, the tube 21 is sealed atthe ends with discs 22 which transmit 280 ,u. light and the tube isfilled with a dry, reasonably inert gas, such as dry argon. The heattransfer block 14' passes through an opening formed in the reflector,said block having a concave end in en gagement with the thermoelectriccooling mechanism 13. The transparent heat shield 12 extendslongitudinally through the reflector and is disposed between the lampand the crystal 11. As noted above, these heat dissipating means may notbe necessary if one of my improved crystals is used and the operatingtemperature does not exceed about 150F.

As noted above I have discovered that thalliumactivated alkali halidecrystals are extremely sensitive to surface contamination, i.e. if thesurface of such a crystal is not scrupulously clean the crystal willrapidly lose its fluorescent properties in use. While the nature of thecontamination which is the apparent causes of rapid deterioration of thefluorscent properties of such crystals has not been investigated, it isknown that certain organic contaminants such as degraded rubber etc.,appear to be especially harmful. Knowledge of the exact nature of theharmful contaminants is not necessary to the practice of this invention,however since it is known that crystals obtained by the usual methods ofprepartion are normally sufficiently contaminated to materially reducetheir useful life, but can be treated according to the method of thepresent invention to produce uncontaminated crystals having greatlyprolonged fluorescent life.

As noted above, the improved uncontaminated crystals of the presentinvention are obtained by first washing ordinary. thallium -activatedalkali halide crystals with any suitable solvent to remove thecontamination. Such solvents include aqueous solvents having solventaction for the underlying alkali halide crystal, and nonaqueous solventshaving solvent action for the contaminants but not the crystal itself.In the preferred method of the crystal is first washed in fresh reagentgrade hexane. The crystals are then dried in air by any suitableconventional technique and washed again with fresh reagent grade methylalcohol. After drying, the decontaminated crystals are handled withclean disposable plastic gloves while fitting them into clean glasstubes; this operation being carried out in an atmosphere of dry argon toprevent recontamination of the crystals. The crystals are so spaced fromthe walls of the glass tubes by suitable spacers, such as rings ofTeflon (Du- Pont trademark for polytetrafluoroethylene), which are inertto the crystals and to ultraviolet radiation. It is important not to userubber or similar spacers, since such materials are degraded byultraviolet radiation and would recontaminate the crystal. The ends ofthe tube containing the decontaminated crystal are then sealed withdiscs of glass or other material transparent to 280 p. wavelengthfluorescent light by any suitable means such as fusion of glass toglass. In order for the fluorescent unit to retain its sensitivity inuse it is essential that the tube be hermetically sealed or air tight.lnasmuch as the assembly operation is carried out in an enclosure filledwith dry argon, the sealed tubes are also filled with this gas, whichprovides a suitably inert atmosphere to inhibit degradation of thecrystal. As noted above decontaminated crystals sealed in sealed tubessuch as these retain their phosphorescent strength indefinitely whenirradiated by 254 p. light at temperatures not in excess of about F. andare far less sensitive to heat and even deleterious p. light thancrystals which have not been cleaned according to the present invention.Therefore, the heat dissipating means and optical filtering meansdescribed above are generally not necessary when these improved, highlystable crystals are used.

When reference is made to thallium, in the foregoing description, it isunderstood that thallium in the 1" va lence state is referred to. Also,it is to be understood that other alkali halide salts, for example,sodium chloride, can be substituted for the described potassiumchloride.

I claim:

1. A source of ultraviolet illumination for use in photometricapparatus, comprising:

primary source means for radiating ultraviolet light in at least onedirection;

fluorescent means, positioned in said one direction from said primarysource: means, for emitting light along at least one path having atleast some wavelengths substantially within the range of about 270 to320 millimicrons upon. receiving said ultraviolet light from saidprimary source means;

said fluorescent means including an article of thallium-activated alkalihalide;

said photometric apparatus including a flow cell positioned in said onepath, whereby light from said fluorescent means is directed into saidflow cell.

2. A source of ultraviolet illumination according to claim 1 in whichsaid fluorescent means includes means for emitting light having asubstantial amount of power at the wavelength of 280 millimicrons uponreceiving said ultraviolet light from said primary source means.

3. A source of ultraviolet illumination according to claim 1 in which:

said article of thallium-activated alkali halide is an elongated crystalhaving at least one rounded end and a longitudinal axis;

said longitudinal axis being transverse to said one direction; and

said rounded end being aligned with said one path,

whereby said emitted light is collimated as it leaves said rounded end.

4. A source of ultraviolet illumination according to claim 1 in whichsaid article of thallium-activated alkali halide comprises from 10 to10" mole fraction of thallium chloride in alkali chloride.

5. A source of ultraviolet illumination according to claim 1 in whichsaid primary source means includes a mercury-vapor lamp radiating lightat least one portion of which has a wavelength of 254millimicrons.

6. A source of ultraviolet illumination according to claim 1 furthercomprising:

an optical filter disposed between said primary source means and saidfluorescent means;

said optical filter transmitting light having a wavelength longer thanabout 220millimicrons and absorbing light having a wavelength shorterthan about 220 millimicrons. I

7. A source of ultraviolet illumination according to claim 1 in which;

said primary source means includes means for shielding said article ofthallium-activated alkali halide from heat; and

said fluorescent means includes means for removing heat from thearticle.

8. A source of ultraviolet illumination according to claim 1 in whichsaid artica] of thallium-activated alkali halide is free from organiccontamination.

9. A source of ultraviolet illumination according to claim 8 in whichsaid fluorescent means includes:

a sealed tube having at least one wall portion that passes light havinga wavelength of substantially 280 millimicrons and at least one wallportion that passes light having a wavelength lower than 280milimicrons;

said article of thallium-activated alkali halide being mounted in saidsealed tube.

10. A source of ultraviolet illumination according to claim 9 in whichsaid fluorescent means further includes:

an inert gas within said sealed tube;

the interior of said sealed tube being free of organic material.

11. A source of ultraviolet illumination according to claim 3 in which:

said article of thallium-activated alkali halide is an elongated crystaland has at least one rounded end and at least one elongated sideportion;

said one rounded end being aligned with said one wall portion thatpasses light having a wavelength of substantially 280 millimicrons; and

said one elongated side portion being aligned with said one wall portionthat passes light having a i wavelength lower than 280 millimicrons. 12.A source of ultraviolet illumination according to claim 11 in which:

said primary source means includes a mercury vapor lamp; and saidmercury-vapor lamp includes means for radiating light, at least oneportion of which has a wavelength of 254 millimicrons. 13. A source ofultraviolet illumination for use in photomeric apparatus. comprising:

primary source means for radiating ultraviolet light in at least onedirection; fluorescent means, positioned in said one direction from saidprimary source means, for emitting light along at least one path havingat least some wavelengths substantially within the range of about 270 to320 millimicrons upon receiving said ultraviolet light from said primarysource means; said fluorescent means including an article ofthalliumactivated alkali halide; said photomeric apparatus including aflow cell positioned in said one path, whereby light from saidfluorescent means is directed into said flow cell: said fluorescentmeans including a reflector having an elliptical cross-section and twofocal axes; said primary source means including a bright spot; saidbright spot being in one of said two focal axes of said reflector; theother of said two focal axes of said reflector passing through a portionof said article of thalliumactivated alkali halide. 14. A source ofultraviolet illumination according to claim 13 in which said article oftallium-activated alkali halide is free from organic contamination.

=l =l= l =l UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OFCORRECTION PATENT N 1 3,887,813 Page 1 0f 3 DATED 1 June 3, 1975INVENTOR( 1 Robert W. Allington It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

, line 10, after 280, change "u" to "mu".

Column 1 Column 1, line 18, after the Word "that", insert the Word "it".Column 1, line 20, after 280, change 1" to "mu".

Column 1, line 24, after 280, change 1" to "mu".

Column 1, line 31, after 280, change 1" to "mu".

Column 1, line 31, after the word "light", omit the first occurrence ofthe Word "either".

Column 1, line 39, after the word "that", change "the" to "a".

Column 1, line 41, after 280, change 1" to "mu" Column 1, line 53, after320, change 1'' to "mu" Column 1, line 58, change the word "lenses" to"lens".

Column 2, line 1, change the word "particularlly" to "particularly".

Column 2, line 1, after 185, change 1" to "mp".

Column 2, line 16, after 220, change 1" to "mu".

Column 2, line 16, change the Word "whicih" to "which".

Column 2, line 18, change both occurrences of "u" to "mu".

Column 2, line 22 and 23, change "thalliumactivated" to"thallium-activated".

Column 2, line 33, change the semicolon to a comma.

Column 2, line 51, change both occurrences of "u" to "mu" Column 3, line21 and 22, change "in corporated" to "incorporated".

Column 3, line 40, after 280, change 1" to "mu" Column 3, line 54,change the word "cary" to "carry".

Column 3, line 59, after 254, change "11" to "mu".

Column 3, line 61, after 320, change 1" to "mu" Column 3, line 63,change the word "flourescent" to "fluorescent".

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT N0 3,887,813 Page 2 of 3 DATED I June 3, 1975 lN\/ ENTOR(5) 1Robert W. Allington It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 4, line 4, after the word "block", insert the word "of".

II II Column 4, line 19, after 320, change u to "mu".

Column 4, line 22, remove the period before the quote marks. Column 4,line 34, after 185, change 1'', to "mu" Column 4, line 35, change theWord "flluorescent" to "fluorescent".

Column 4, line 39, after 254, change "u", to "mu Column 4, line 42,after 185, change "u", to "mu". Column 4, line 47, after 185, change 1to "mp". Column 4, line 49, after 220, change to "mu". Column 4, line 49and 50, after 220, change 1" to "mu". Column 4, line 50, change"highsilica" to "high-silica".

0 Column 4, line 68, change the word "utlized" to "utilized".

Column 5, line 7, after 185, change "u", to "mu" Column 5, line 9,change the word "abovementioned" to "above-mentioned" Column 5, line 10,after 220, change "1.1", to "mu".

. Column 5, line 11, after 220, change "11'', to "mp". Column 5, line12, after 280, change "11'', to "mp". Column 5, line 29, change the word"causes" to "cause". Column 5, line 35, after the word "however", placea comma. Column 5, line 59, after the word "are", omit the word "so".Column 5, line 68, after 280, change 1'', to "mu".

0 Column 6, line 10, after 254, change .1", to "mu". Column 6, line 12,after 185, change 1'', to "mu". Column 6, line 68, after the semicolon,add the Word "and". Column 7, line 21, after the semicolon, add the word"and". Column 7, line 27, after the semicolon, add the word "and".

T Column 8, line 6, place a hyphen between the words "mercury" d"vapor".

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT I 3 887 813 Page 3 of 3 DATED I June 3, 1975 INV.ENTOR(S) 1Robert W. Allington ltis certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 8, line 12, after the word "apparatus", remove the period.

Column 8, line 21 and 22, change "thalliumactivated" to"thallium-activated". a

Column 8, line 25, change the colon to a semicolon.

Column 8, line 25, a new paragraph should begin with the word "said".

Column 8, line 30, after the semicolon, add the word "and".

Column 8, line 35, change the word "tallium-activated" to"thallium-activated".

Signed and Sealed this Form]: D3) r December 1979 [SEAL] Anest:

SIDNEY A. DIAMOND Arresting Oflicer Commissioner of Patents andTrademarks

1. A SOURCE OF ULTRAVIOLET ILLUMINATION FOR USE IN PHOTOMETRICAPPARATUS, COMPRISING: PRIMARY SOUCE MEANS FOR RADIATING ULTRAVIOLETLIGHT IN AT LEAST ONE DIRECTION; FLUORESCENT MEANS, POSITIONED IN SAIDONE DIRECTION FROM SAID PRIMARY SOURCE MEANS, FOR EMITTING LIGHT ALONGAT LEAST ONE PATH HAVING AT LEAST SOME WAVELENGTH SUBSTANTIALLY WITHINTHE RANGE OF ABOUT 270 TO 320 MILLIMICRONS UPON RECEIVING SAIDULTRAVIOLET LIGHT FROM SAID PRIMARY SOURCE MEANS; SAID FLUORESCENT MEANSINCLUDING AN ARTICLE OF THALLIUMACTIVATED ALKALI HALIDE; SAID PHOTOMETICAPPARATUS INCLUDING A FLOW CELL POSITIONED IN SAID ONE PATH, WHEREBYLIGHT FROM SAID FLUORESCENT MEANS IS DIRECTED INTO SAID FLOW CELL.
 1. Asource of ultraviolet illumination for use in photometric apparatus,comprising: primary source means for radiating ultraviolet light in atleast one direction; fluorescent means, positioned in said one directionfrom said primary source means, for emitting light along at least onepath having at least some wavelengths substantially within the range ofabout 270 to 320 millimicrons upon receiving said ultraviolet light fromsaid primary source means; said fluorescent means including an articleof thallium-activated alkali halide; said photometric apparatusincluding a flow cell positioned in said one path, whereby light fromsaid fluorescent means is directed into said flow cell.
 2. A source ofultraviolet illumination according to claim 1 in which said fluorescentmeans includes means for emitting light having a substantial amount ofpower at the wavelength of 280 millimicrons upon receiving saidultraviolet light from said primary source means.
 3. A source ofultraviolet illumination according to claim 1 in which: said article ofthallium-activated alkali halide is an elongated crystal having at leastone rounded end and a longitudinal axis; said longitudinal axis beingtransverse to said one direction; and said rounded end being alignedwith said one path, whereby said emitted light is collimated as itleaves said rounded end.
 4. A source of ultraviolet illuminationaccording to claim 1 in which said article of thallium-activated alkalihalide comprises from 10 4 to 10 6 mole fraction of thallium chloride inalkali chloride.
 5. A source of ultraviolet illumination according toclaim 1 in which said primary source means includes a mercury-vapor lampradiating light at least one portion of which has a wavelength of254millimicrons.
 6. A source of ultraviolet illumination according toclaim 1 further comprising: an optical filter disposed between saidprimary source means and said fluorescent means; said optical filtertransmitting light having a wavelength longer than about 220millimicrons and absorbing light having a wavelength shorter than about220 millimicrons.
 7. A source of ultraviolet illumination according toclaim 1 in which: said primary source means includes means for shieldingsaid article of thallium-activated alkali halide from heat; and saidfluorescent means includes means for removing heat from the article. 8.A source of ultraviolet illumination according to claim 1 in which saidartical of thallium-activated alkali halide is free from organiccontamination.
 9. A source of ultraviolet illumination according toclaim 8 in which said fluorescent means includes: a sealed tube havingat least one wall portion that passes light having a wavelength ofsubstantially 280 millimicrons and at least one wall portion that passeslight having a wavelength lower than 280 milimicrons; said article ofthallium-activated alkali halide being mounted in said sealed tube. 10.A source of ultraviolet illumination according to claim 9 in which saidfluorescent means further includes: an inert gas within said sealedtube; the interior of said sealed tube being free of organic material.11. A source of ultraviolet illumination according to claim 3 in which:said article of thallium-activated alkali halide is an elongated crystaland has at least one rounded end and at least one elongated sideportion; said one rouNded end being aligned with said one wall portionthat passes light having a wavelength of substantially 280 millimicrons;and said one elongated side portion being aligned with said one wallportion that passes light having a wavelength lower than 280millimicrons.
 12. A source of ultraviolet illumination according toclaim 11 in which: said primary source means includes a mercury vaporlamp; and said mercury-vapor lamp includes means for radiating light, atleast one portion of which has a wavelength of 254 millimicrons.
 13. Asource of ultraviolet illumination for use in photomeric apparatus.comprising: primary source means for radiating ultraviolet light in atleast one direction; fluorescent means, positioned in said one directionfrom said primary source means, for emitting light along at least onepath having at least some wavelengths substantially within the range ofabout 270 to 320 millimicrons upon receiving said ultraviolet light fromsaid primary source means; said fluorescent means including an articleof thalliumactivated alkali halide; said photomeric apparatus includinga flow cell positioned in said one path, whereby light from saidfluorescent means is directed into said flow cell: said fluorescentmeans including a reflector having an elliptical cross-section and twofocal axes; said primary source means including a bright spot; saidbright spot being in one of said two focal axes of said reflector; theother of said two focal axes of said reflector passing through a portionof said article of thallium-activated alkali halide.